Origins of descending spinal pathways in prehensile birds: do parrots have a homologue to the corticospinal tract of mammals?
In mammals, the supraspinal descending projections that influence distal limb muscles are the rubrospinal and corticospinal tracts. The former, which is found in other vertebrates, shows greater somatotopy in mammals that are 'dextrous' (e.g. monkeys) than those that are not (e.g. opossums). Similarly, the corticospinal tract, which is found only in mammals, has more extensive connections (i.e. direct corticomotoneural) in mammals that are dextrous than in mammals that are not. Descending spinal pathways have been described in 'non-dextrous' avian species (chickens, ducks, geese and pigeons), and the purpose of this study was to determine if there are any differences in the origins of descending projections to the spinal cord in 'dextrous' or prehensile parrots (sulphur-crested cockatoo, Cacatua galerita, and eastern rosella, Platycerus eximius). True Blue or wheat germ agglutinin-horseradish peroxidase was injected into the lumbar or cervical spinal cord. The distribution of retrogradely labelled cells was similar to that previously reported for non-prehensile birds. We found no evidence of any direct spinal projections from the telencephalon (including any pathway homologous to the corticospinal tract of mammals), nor any specialized anatomical organization of the descending pathways that could account for the pedal dexterity of these species.
Webster
D M
DM
School of Rehabilitation Medicine, University of British Columbia, Vancouver, Canada.
Rogers
L J
LJ
Pettigrew
J D
JD
Steeves
J D
JD
D000818:Animals; D005075:Biological Evolution; D001931:Brain Mapping; D001933:Brain Stem / Q000033:anatomy & histology; D004292:Dominance, Cerebral / Q000502:physiology; D005260:Female; D008297:Male; D009474:Neurons / Q000648:ultrastructure; D010311:Parrots / Q000033:anatomy & histology*; D011597:Psychomotor Performance / Q000502:physiology*; D011712:Pyramidal Tracts / Q000033:anatomy & histology*; D013045:Species Specificity; D013116:Spinal Cord / Q000033:anatomy & histology*